1. Field
Example embodiments generally relate to an image forming apparatus using an electrophotographic method, such as a copying machine, a printer, a facsimile machine, and a multifunction apparatus that combines the functions of the copying machine, the printer, and the facsimile machine, and a fixing device installed in the image forming apparatus, for example, a fixing device using a belt fixing system including a fixing belt.
2. Description of the Related Art
A related-art image forming apparatus, such as a copying machine, a facsimile machine, a printer, or a multifunction printer having two or more of copying, printing, scanning, and facsimile functions, forms a toner image on a recording medium (e.g., a sheet) according to image data using an electrophotographic method. In such a method, for example, a charger charges a surface of an image carrier (e.g., a photoconductor). An optical device emits a light beam onto the charged surface of the photoconductor to form an electrostatic latent image on the photoconductor according to the image data. The electrostatic latent image is developed with a developer (e.g., a toner) to form a toner image on the photoconductor. A transfer device transfers the toner image formed on the photoconductor onto a sheet. A fixing device applies heat and pressure to the sheet bearing the toner image to fix the toner image onto the sheet. The sheet bearing the fixed toner image is then discharged from the image forming apparatus.
In related-art image forming apparatuses, a fixing device using a belt fixing system, which includes a fixing belt serving as a fixing member, is widely used. In one example, the fixing device includes a heat roller in which a heater is provided, a fixing belt tightly stretched across a plurality of rollers such as the heat roller, a pressing roller for pressing against the heat roller via the fixing belt to form a nip portion, and so forth. A recording medium is conveyed through the nip portion formed between the fixing belt and the pressing roller, so that a toner image is fixed onto the recording medium. Since the fixing device using the belt fixing system includes the fixing member with a lower heat capacity as compared to a fixing member included in a fixing device using a roller fixing system, a rise time of the fixing device can be shortened, resulting in greater printing efficiency. However, one problem with such an arrangement is that a temperature overshoot in the fixing member may occur after printing has been completed.
A temperature overshoot occurs because the fixing belt has a small heat capacity, and therefore a portion thereof heated by the heat roller has the highest temperature whereas temperatures of other portions thereof easily decrease. Specifically, the fixing belt has a temperature distribution in a circumferential direction in which the portion heated by the heat roller has the highest temperature, giving rise to a relatively large temperature difference between an inner circumferential surface of the heat roller facing the heater and a surface of the fixing belt. The foregoing temperature distribution becomes more pronounced and the temperature difference increases further the longer printing continues.
When driving of the fixing belt is stopped after printing has been completed, the fixing device enters a standby state to keep the temperature thereof lower than that during printing. Under such temperature distribution conditions with its large temperature differences, the heat of the heat roller is transferred to the fixing belt to maintain heat balance. Moreover, the heater is turned on whenever the temperature of the fixing belt is lower than a desired temperature while the fixing belt is stopped. As a result, heat from the heater is further added to the fixing belt, so that the temperature of the fixing belt becomes considerably higher than the desired temperature during standby, causing temperature overshoot.
There is an additional concern, insofar as components of the fixing device are repeatedly heated at a higher temperature due to such temperature overshoot, and consequently secondary problems, such as deterioration of a rubber layer included in the fixing belt and a fixing auxiliary roller, detachment of an adhesive layer included in rollers having a plurality of layers, metal fatigue of the heat roller, and so forth, may occur.
One possible method of solving the above-described problems is to idle the fixing belt after printing has been completed. However, when the fixing belt is idled at a higher speed, the temperature distribution and the temperature difference described above may be aggravated due to a further release of the heat from the fixing belt.
Another example of a fixing device is proposed in which a driving speed of a fixing belt is changed based on toner type and toner image resolution. However, the object of such a fixing device is to obtain a preferred fixing performance, and not to solve the above-described problems.
Yet another example of a fixing device using a temperature control method is proposed in which, by using a contactless temperature sensor, a desired temperature is changed at predetermined intervals so that temperature overshoot or undershoot relative to the desired temperature can be reduced. However, one drawback of such a fixing device employing the contactless temperature sensor is that temperature readings sometimes lack the precision in a short time required for high-speed image formation. The object of such a fixing device is to solve problems specific to the fixing device using the contactless temperature sensor.